Early Archean biogeochemical iron cycling and nutrient availability: New insights from a 3.5 Ga land-sea transition

The chemical and isotopic compositions of Precambrian Fe-rich chemical sedimentary rocks have figured prominently in discussions on the Fe biogeochemical cycle and redox conditions in the early Earth. Broad trends of decreasing δ⁵⁶Fe values for Eoarchean to Paleoproterozoic iron formations (IFs) and jaspilites (hematite-chert) with decreasing age reflect a general increase in extent of oxidation of aqueous Fe(II) (Fe(II)_aq) in marine environments, consistent with increasing oxygenation of surface environments, particularly since the Mesoarchean. Such trends may record a shift from anaerobic to aerobic oxidation, in part reflecting increasing nutrient abundances that follow increased emergence of continental crust. It is commonly proposed that the size of the biosphere in the early Archean was largely nutrient limited, specifically P limited. Highly positive δ⁵⁶Fe values for Fe oxides in chemical sedimentary rocks deposited in open marine environments at this time are consistent with excess Fe(II)_aq. Here, we present new data on Fe isotopes and trace element chemistry for 3.4–3.5 Ga jaspilites from the North Pole region of the Pilbara Craton, including the first example of a land-sea transition preserved in the lower Dresser Formation as jaspilite deposition in restricted basin and open-marine settings. A review of the possible mechanisms of Fe(II)_aq oxidation in the early Archean - including O₂ generated by oxygenic photosynthesis, UV photo-oxidation, or photoferrotrophy (anoxygenic photosynthesis) - suggests that the latter is most likely, in light of evidence for low-O₂ contents and the strong role that silica plays in inhibiting abiologic Fe(II) oxidation. Y + REE contents of lower Dresser Formation jaspilites indicate a freshwater component in the restricted basin settings, in contrast to exclusively seawater components in the open-marine environments. δ⁵⁶Fe values for open-marine jaspilites are highly positive (δ⁵⁶Fe ~ +1 to +2.4‰), similar to other jaspilites and IFs of Eoarchean to Paleoarchean age. Assuming photoferrotrophy as the oxidative pathway, this would indicate electron donor (Fe(II)_aq) excess and nutrient limitation. For the restricted-basin jaspilites of the lower Dresser Formation, δ⁵⁶Fe values extend to slightly negative values (δ⁵⁶Fe ~ −0.4‰), correlating with changes in Y + REE contents that indicate fluid mixing. In toto, there is a range of ~3‰ in δ⁵⁶Fe values across the land-sea transition of the lower Dresser Formation, significantly exceeding the range previously measured in early Archean jaspilites. Combined with an advection-dispersion-reaction model for Fe(II)_aq oxidation via photoferrotrophy, these observations suggest a range of nutrient abundance with environment, where samples from near-shore or restricted settings record excess nutrients from terrestrial input and hence electron donor limitation. Integrating the new results from the Pilbara Craton with the broader Fe isotope database for Eoarchean through Paleoarchean jaspilites and IFs, as well as a simple mass-balance model relating δ⁵⁶Fe values and P contents, it becomes apparent that the relatively high nutrient abundance seen only in restricted settings at ~3.5 Ga becomes more characteristic of the open oceans by ~3.2 Ga. This expansion of nutrient availability is correlated with a marked increase in ⁸⁷Sr/⁸⁶Sr ratios for seawater, which can be shown to be a proxy for P delivery to the open oceans from the continents via weathering fluxes. Collectively, these findings demonstrate the importance of considering geologic context in interpreting Fe isotope and chemical compositions of jaspilites and IFs. Capture of the land-sea transition of the lower Dresser Formation provides insights into the biosphere that would have been missed in sample suites that only record open-marine conditions.